V-pattern exotropia is defined as 15 PD or greater exotropia in up gaze than in down gaze, which is usually associated with IO overaction. IO muscle weakening is used to improve the pattern . V-pattern exotropia strabismus is common in craniosynostosis children, with as many as two thirds of patients manifesting the condition. Anatomic changes to the orbit in craniosynostosis have been postulated to result in relative sagittalization of the origin of the IO, causing the application of Hering’s law to extraocular muscles that are anatomically excyclorotated in craniosynostosis patients [6,9]. Another theory considered that IO overaction results from enhanced contact of the IO with the floor of the globe. A number of publications report on the absence of muscles and attribute to the atypical patterns of strabismus. Craniosynostosis is a complex disorder producing complicated strabismus, which is very difficult to correct surgically.
We found that there was no significant difference in the age of strabismus operation between children with craniosynostosis and normal children, but the duration of strabismus was longer, suggesting that the age of strabismus onset was earlier. There is no definite conclusion about the sequence of strabismus surgery and craniofacial surgery in craniosynostosis children , but cranioplasty surgery must be performed first to relieve intracranial hypertension.
Comparing with the control group, the deviation in primary and up gaze was larger. A larger V pattern was significant in craniosynostosis, which was consistent with the IO overaction and SO underaction. We proposed that these were the characteristic of V-pattern strabismus in craniosynostosis, a larger V sign and large deviation due to abnormal orbital anatomical structure.
Different surgical techniques have been used on strabismus in craniosynostosis. Several studies have examined the best way to correct the V-pattern and over-elevation in adduction [6,10-12].This problem is complex and difficult to cure with surgery. Denervation/extirpation and myectomy of the IO muscle offered modest benefits, though neither procedure resulted in normalization of ocular motility. Transposition of the rectus muscles in combination with weakening of the oblique muscles is effective. SO tuck may also provide effective reversal excyclotorsion.
In this study, we used the IO muscle anterior transportation combined with horizontal strabismus surgery to correct the V pattern. For the same deviation, the lateral recession was larger in craniosynostosis than that in the control group, suggesting muscles were weaker in craniosynostosis. In the IO muscle anterior transportation, the transportation in craniosynostosis was more anterior than control group. During the surgery, it was difficult to hook the IO muscle, because the position was lower because of the narrow orbit and proptosis in craniosynostosis. We found that the extraocular muscles were thinner and weaker in craniosynostosis children, and the IO muscle was surrounded by fat tissues. Extraocular muscle abnormalities was more common in craniosynostosis children.
In the past decades, significant progress has been made in understanding the genetic basis of craniosynostosis with mutations in the fibroblast growth factor (FGF) signaling pathway. They are important in neuronal differentiation, angiogenesis, wound healing, limb development, and mesoderm induction [13-14]. In strabismus patients, the molecular composition of extraocular muscles was altered, including myosins, tropomyosins, troponins, and collagen related proteins . However, there are few studies on the molecular and protein difference between craniosynostosis and normal extraocular muscles.
We used to report the management of V-pattern strabismus with marked inferior rectus loss in craniosynostosis, and there were pathological changes and collagen degeneration in extraocular muscles . In this study, proteomic analysis of extraocular muscles was performed. According to the classification of different proteins in extraocular muscles, we found that the increased protein functions including catalytic activity, metal ion binding, and protein binding. The decreased protein functions including protein binding and catalytic activity. Our study found that troponin C, myosin regulatory light polypeptide 9, and collagen alpha-1 chain were higher in the extraocular muscles of craniosynostosis group than in the control group. Troponin was contraction-related proteins. Myosins were instrumental in regulating contraction force and velocity of muscle fibers . However, they were reported downregulated in strabismic muscles . We speculate this is related to the abnormal development of craniosynostosis.
We compared the differences of V-pattern exotropia, surgery characteristics, and muscle proteins between craniosynostosis group and the control group. A larger V pattern and larger deviation is common in craniosynostosis children. For the same PD of deviation, it usually needs larger recession in craniosynostosis because of the thinner and weaker extraocular muscles. And extraocular muscle abnormalities was more common in craniosynostosis children.
The limitation is that more sample and follow-up are needed. We will continue to study in our future work.